[Rd] gsub() hex character range problems in R-devel?

Tomas Kalibera
Wed, 05 Jan 2022 09:17:01 -0000

Hi Martin,

I'd add few comments to the excellent analysis of Brodie.

- \xhh is allowed and defined in Perl regular expressions, see ?regex
(would need perl=TRUE), but to enter that in an R string, you need to
escape the backslash.

- \xhh is not defined by POSIX for extended regular expressions, neither
it is documented in ?regex for those; TRE supports it, but still
portable programs should not rely on that

- literal \xhh in an R string is turned to the byte by R, but I would
say this should not be used at all by users, because the result is
encoding specific

- use of \u and \U in an R string is fine, it has well defined semantics
and the corresponding string will then be flagged UTF-8 in R (so e.g.
\ua0 is fine to represent the Unicode no-break space)

- see caveats of using character ranges with POSIX extended regular
expressions in ?regex re encodings, using Perl regular expressions in
UTF-8 mode is more reliable for those

So, a variant of your example might be:

 > gsub("[\\x7f-\\xff]", "", "fo\ua0o",
perl=TRUE)
[1] "foo"

(note that the \ua0 ensures that the text is UTF-8, and hence the UTF-8
mode for regular expressions is used, ?regex has more)

However, I think it is better to formulate regular expressions to cover
all of Unicode, so do something like e.g. "only keep ASCII digits, ASCII
space, ASCII underscore, but remove all other characters".

Best
Tomas

On 1/4/22 8:35 PM, Martin Morgan wrote:

> I'm not very good at character encoding / etc so this might be user
error. The following code is meant to replace extended ASCII characters, in
particular a non-breaking space, with "", and it works in R-4-1-branch
>
>> R.version.string
> [1] "R version 4.1.2 Patched (2022-01-04 r81445)"
>> gsub("[\x7f-\xff]", "", "fo\xa0o")
> [1] "foo"
>
> but fails in R-devel
>
>> R.version.string
> [1] "R Under development (unstable) (2022-01-04 r81445)"
>> gsub("[\x7f-\xff]", "", "fo\xa0o")
> Error in gsub("[\177-\xff]", "", "fo\xa0o") :
invalid regular expression '[-?]', reason 'Invalid character
range'
> In addition: Warning message:
> In gsub("[\177-\xff]", "", "fo\xa0o") :
>    TRE pattern compilation error 'Invalid character range'
>
> There are other oddities, too, like
>
>> gsub("[[:alnum:]]", "", "fo\xa0o")  #
R-4-1-branch
> [1] "\xfc\xbe\x8c\x86\x84\xbc"
>
>> gsub("[[:alnum:]]", "", "fo\xa0o")  #
R-devel
> [1] "<>"
>
> The R-devel sessionInfo is
>
>> sessionInfo()
> R Under development (unstable) (2022-01-04 r81445)
> Platform: x86_64-apple-darwin19.6.0 (64-bit)
> Running under: macOS Catalina 10.15.7
>
> Matrix products: default
> BLAS:   /Users/ma38727/bin/R-devel/lib/libRblas.dylib
> LAPACK: /Users/ma38727/bin/R-devel/lib/libRlapack.dylib
>
> locale:
> [1] en_US.UTF-8/en_US.UTF-8/en_US.UTF-8/C/en_US.UTF-8/en_US.UTF-8
>
> attached base packages:
> [1] stats     graphics  grDevices utils     datasets  methods   base
>
> loaded via a namespace (and not attached):
> [1] compiler_4.2.0
>
> (I have built my own R on macOS; similar behavior is observed on a Linux
machine)
>
> Any hints welcome,
>
> Martin Morgan
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